Green Man Publishing

Sweet Potato

Ipomoea batatas

Introduction: The sweet potato is native to the Americas and was introduced into Europe very early in the history of European colonization (about 1500). Many of the earliest references to the potato probably actually mean this species. Predictably it didn’t do very well in chilly Northern Europe, but was taken from there to India, China and other tropical countries where it became an important food crop.

Sweet potato should not to be confused with the yam, which is a member of the Dioscoreaceae family. It is actually a relative of the morning glory, which is apparent if you see their flowers.

Crop use: In the right climate the sweet potato is one of the most nutritious and productive vegetables you can grow. In places that are too hot to grow potatoes it often takes their place as the staple vegetable crop. It can be an important crop for those aiming for food self-sufficiency in warm climates.

Ease of growing: In a suitably warm climate the sweet potato is pretty easy to grow, though it is vulnerable to quite a few pests and diseases.

Climate: Sweet potato is actually a perennial in tropical climates, but in temperate regions it can’t survive the cold winters and so must be grown as an annual.

It does best in a hot, humid climate with a long growing season (ideally 4 – 5 months of warm or hot weather) and moderate, evenly distributed rainfall. For best quality it needs warm nights (a minimum of 60˚F and ideally over 70˚F) as well as warm days. In the United States it is most widely grown in the southeastern states.

The sweet potato can also be grown in less than ideal conditions, but you have to work at it and give it some protection in cool weather. I’ve done it a couple of times in my garden, but the nights are too cool for it to be very productive.

About Sweet potato  
Planning facts
Perennial
Hardiness: Tender
Growing temp: 65 (70 – 85) 95˚F
Plants per person: 5
Plants per sq ft: 1
Weeks to grow transplants: 8 – 12
Plant out: 4 weeks after last frost
Days to harvest: 130 – 170  

Harvest facts
Yield per plant: 4 oz – 3 lb
Yield per sq ft: 1 – 3 lb  

Nutritional content: Sweet potato is an important food crop from a nutritional standpoint. It is a good source of vitamin C and B6, while those with orange flesh are rich in vitamin A (beta carotene). It is also a good source of calcium, potassium and pectin, as well as some beneficial phytochemicals including lutein and zeaxanthin.

Site: Sweet potato is a fast growing and vigorous vine and most types requires quite a lot of space, though there are a few more compact bush varieties. All need as much sun as they can get.

The plants are quite vulnerable to pests and diseases, so should be rotated regularly. Don’t grow them in the same place again for at least 3 years.

 Soil

pH 4.5 (5.5 ‑ 6.5)7.0

Sweet potato can be grown on soil that is too poor for most crops, though (as is usually the case) better soil will give you a larger crop. The ideal soil for sweet potatoes is a rich, deep, well-drained, sandy but moisture retentive loam. It should be fairly acidic, as disease problems are more common in neutral soils. Clay soil can produce slender roots, while poorly drained soils can result in root disease.

Soil preparation: Incorporate 2 – 4˝ of compost or aged manure into the top 6 – 10˝ of soil. No matter what type of soil you have, adding organic matter will be beneficial when growing sweet potatoes. It lightens a heavy clay soil and improves its texture. It helps a sandy soil to retain moisture (and improves its texture too). Double digging is a good idea if the soil is heavy.

Like most root crops sweet potatoes need phosphorus (colloidal phosphate is good) and potassium (add greensand or wood ashes). They don’t need a lot of nitrogen, as it encourages the growth of foliage rather than roots (and can result in inferior quality roots).

Ridges: When growing sweet potatoes in cooler areas, the soil is often shaped into ridges, as these warm up (and drain) faster than flat soil. This doesn’t always work in hotter areas, or with very sandy soil though, as they tend to dry out too quickly. 

Lighter soil can be shaped into ridges 6 – 8˝ high and 12˝ wide, with 30˝ between the ridges. On heavy soil they may be 12 – 15˝ wide and 36 – 48˝ apart. Raised beds also work well.

Planning

Where: These creeping tropical vines need plenty of sunlight and a lot of growing space. In cooler climates they must be planted in the warmest and most sheltered spot you have. They should also be planted where they can sprawl without interfering with other crops.

When: Sweet potatoes are tropical plants and don’t tolerate cold very well, so there is no point in trying to get them in the ground early. Don’t plant them out until at least 3 – 4 weeks after the last frost date and the soil is at least 60˚F (65˚F or higher is better). If you need to gain some extra time you can warm up the soil with black plastic for a couple of weeks before planting. The roots need to be harvested before the soil temperature drops down to 55˚F in fall.

Buying plants: In many areas of the country slips aren’t readily available and you may have to buy them by mail order.

Planting

Sweet potatoes are propagated vegetatively by means of shoots taken from the tubers, known as slips. Whole tubers aren’t usually planted because they send out too many shoots which would eventually compete with each other.

Though sweet potatoes are usually fairly easy to grow, I have encountered a problem in obtaining planting material. In traditional sweet potato growing areas (the southeast) they can be purchased from nurseries and in most other areas they can be bought by mail order (these don’t always arrive in the best shape, but usually do fine).

My problem in California is that you can’t buy them locally and you can’t easily buy them by mail order from another state because of quarantine restrictions (though there are some exceptions). I have got around this problem by raising my own slips from tubers I buy at the store. There is some danger of introducing virus diseases when using shop tubers though and it is better to use certified disease free slips where possible.

Starting your own slips: It is pretty easy to grow your own slips if you have some unsprayed tubers. The best source of tubers are those you have grown yourself, so it is a good idea to save a few of the best ones (they should be fairly thick, at least 1 ½˝ in diameter).

You can also use organic tubers from the supermarket or health food store. Avoid conventional tubers as they are commonly sprayed with a sprout inhibitor, in which case they won’t sprout.

Indoors: Start sprouting the tubers 2 – 3 months before you want to plant them out. The usual way to do this is to half bury them in a bed of damp sand or peat moss (some people cover them with plastic to retain moisture). They should be kept at a temperature of 70 – 90˚F (the warmer it is the faster they will sprout). In 4 – 8 weeks they should produce a number of vigorous shoots with healthy roots on them. You should be able to get 10 – 12 slips per tuber.

Commercial operations sprout the tubers with bottom heat and you might want to try this. In the old days this was done in hot beds filled with fresh manure, covered with a few inches of sand or soil. Now it is done with electric soil heating cables.

You can also suspend a tuber half in water with toothpicks (like an avocado pit). Just make sure the top (flat end) is upright. I haven’t had any luck with this method, as the tuber rotted before it sprouted. Also they don’t tend to have as good a root system.

When the slips are large enough they are carefully detached from the tuber. You can remove individual slips as they get big enough and leave the tuber to produce more.

If you detach the slip with all of its root attached it will recover faster, but you will transfer any disease from the tuber. If you cut it an inch away from the tuber it will take longer to recover, but you may leave any disease behind. You can put these in 4˝ pots until they grow roots and then plant out.

If it’s still too cold outside, you might have to transplant them into bigger (1 gallon) pots, or a deep flat.

Outdoors: If it is warm enough you can start slips outdoors (or under cloches or cold frames) following the procedure described above.

Planting out: I prefer to plant the slips in individual holes. Make a large hole, add a couple of handfuls of compost, along with some wood ashes. Then bury most of the slip, leaving just the tip sticking out and make a slight well to hold water. If the weather is still cool, cover with cloches to keep them warm and happy. If the weather is hot make sure you give them plenty of water until they are settled in.

In some areas you may have to use cutworm collars to protect the newly planted slips.

Spacing: Sweet potatoes are commonly planted in rows, as it is a lot easier to hill them up. Space the plants 12 – 18˝ apart, in rows 36 – 48˝ apart. Bush types can be spaced somewhat closer.

Care

These plants are quite independent and don’t really need much care once they are established.

Water: Established plants are quite tolerant of drought, but the soil should be kept evenly moist (at least 1˝ per week) for maximum production. If water is in short supply then just water while they are young.

Irregular watering (too wet, too dry, too wet) can cause the tubers to crack).

Too much water isn’t a good idea, as it encourages foliage growth rather than root growth.

Weeds: Sweet potatoes are vigorous plants and weeds aren’t a big problem once they get established, but you will need to weed while they are young. If they have to struggle against a lot of weeds during early growth it can reduce the harvest by as much as 20%.

The plants should be weeded 2 – 3 weeks after they go in the ground (and again 3 – 4 weeks later). If you are using a hoe, don’t go too deep as it’s easy to damage the shallow roots. You will also remove weeds later on when you hill the plants.

A layer of mulch will help to keep weeds under control (but makes it hard to hill them).

Fertilization: It isn’t necessary to feed the plants, unless the soil is very poor, in which case give them a feed of compost tea when the tubers are developing. Don’t over‑feed, as this can encourage lush foliage growth at the expense of tuber production.

Mulch: In cool climates a black plastic mulch is often used to give the plants extra heat. The slips are planted through slits in the plastic.

In hot climates an organic mulch may be used to keep down weeds and conserve soil moisture. Don’t apply this until the soil has warmed up nicely though.

Hilling: As soon as the plants are making good healthy growth they should be hilled up as this can increase the final yield. Of course this is impractical if you are using black plastic mulch, or if they are not planted in rows.

Problems

Frost: Sweet potatoes are very intolerant of cold and any frost will kill the tops.

If a late spring frost threatens, you should cover your plants with frost blankets or a few inches of loose mulch (and maybe a cloche or bucket for extra insurance). This is usually easy because the plants are pretty small at this point.

If an early fall frost threatens, it pays to protect them with straw mulch, cloches or even sheets. This is extra work but you may be rewarded with several more weeks of growing weather. The plants grow rapidly towards the end, so there can be a big benefit in leaving them in the ground as long as possible. When a hard frost threatens, dig the tubers immediately. Don’t wait too long as any damage to the tubers will affect their storability.

Pests: A lot of pests may potentially attack sweet potatoes, but generally they are so vigorous this isn’t a problem (just make sure they get all of the nutrients they need).

One of the few advantages of growing them in the cooler north is that there are less pests to deal with.

Pests include: cucumber beetles, cutworms, flea beetles, Japanese beetles, nematodes, sweet potato leaf beetles, sweet potato weevils, tortoise beetles, wireworms and more.

Diseases: Sweet potatoes are vulnerable to several diseases, including black rot, fusarium wilt, internal cork, pox, scurf, soft rot, stem rot and various fungal, bacterial and viral pestilences.

Use resistant varieties, certified disease free slips, good sanitation, rotation and careful storage to minimize disease problems (some diseases develop during storage). Most diseases are less problematic in the north and in acidic soils.

Sweet potatoes are vulnerable to several diseases in storage, so it is important to harvest carefully (without damaging the skins) and cure properly.

Harvesting

When: You can start harvesting and eating the tubers as soon as they reach a useful size. However if you want to store them you must wait for them to mature fully (the lower leaves will start to turn yellow and die back

Sweet potatoes continue to grow and get bigger until cold weather kills the tops, so it is a common practice to wait until frost threatens and then harvest. If a forecast warns of severe frost you must harvest before it hits, as it can damage any tubers lying near to the surface). Also harvest before the soil drops to 50°F as this can result in internal damage. Don’t worry if a lot of the tubers are small, they will still taste good.

How: The tubers tend to grow vertically and cluster directly underneath the plant (they don’t generally spread out a lot). You will have to dig deep to get them whole, without breaking them.

Always handle the tubers gently as they are easily damaged and even a small wound can lead to decay (some gardeners even line the harvesting container with soft cloth).

Let the newly harvested tubers air dry for a couple of hours and then brush off excess soil. Don’t leave them exposed to the sun for more than an hour though, as this can damage them (some people wait until it is cloudy before harvesting).

The next step is to sort the roots into three piles. The first one for seriously damaged tubers, which should be eaten immediately (though they won’t taste as good if not cured) The second pile is for slightly damaged tubers, which should be eaten fairly promptly. The final pile is for undamaged tubers which can be stored. 

Curing: The flavor of the tubers becomes sweeter after curing, as this encourages the formation of an enzyme which converts some of their starch into sugar. It also heals minor injuries.

Remove any excess soil clinging to the roots before curing, but don’t wash them. Cure the tubers by keeping them at 80 – 90˚F (with 85 – 90% humidity) for 10 days. If the temperature is only 70˚F then leave them for 2 – 3 weeks.

Storage: Store the tubers in paper lined boxes in a dark, humid, well ventilated place at 50 – 60˚F (they may rot if it’s colder than 50˚F). This is not only the best temperature range for storage, but also encourages the conversion of starch to sugar, so they taste better.

Sweet potatoes don’t like being stored at very low temperatures and can be damaged if kept in the fridge for a long period.

Even under the best conditions sweet potatoes don’t store for very long, If treated well they should keep in good condition for several months (but not much longer). They often decay in storage though, so it’s good to check them frequently for rot or deterioration.

Seed saving:

Sweet potatoes rarely flower outside the tropics and even more rarely set seed, so it isn’t used for propagation.

Save some of the best tubers for propagating slips. These should be stored as described above, but even more carefully, as they need to keep longer.

Unusual growing ideas

Cuttings: If you have a long, warm growing season you can propagate sweet potatoes by taking green cuttings from growing plants and rooting them in water.

Indoor plants: Sweet potato vines have been used as indoor plants, though I can’t imagine they produce very much when grown in this way.

Container growing: The bush varieties have been successfully grown in containers. They are also well suited to grow bags, just be sure to use a fertile mix (see Potato for more on this).

Animal food: In the south a significant proportion of the sweet potato crop is used to feed livestock.

Growing in cooler areas: Though sweet potatoes do best where summers are long and warm, it is possible to grow them in colder areas, if you are prepared to do some extra work. You need to use a fast maturing variety, warm the soil with plastic mulch and maybe even use cloches to keep the plants warmer.

Interplanting: Sweet potatoes work well with corn, just give it a couple of weeks head start.

Overwintering: Take some of the smaller tubers from your harvest and pot them up for the winter. Keep these in the greenhouse or windowsill and the following spring you can separate out the new shoots that emerge and plant them in individual pots. When the right time approaches in spring just plant them out.

Other uses: Sweet potatoes can be used as a temporary groundcover (or a permanent one in tropical areas).

They can also be used as a soil improving crop to loosen clay soil.

Varieties

Sweet potatoes may be divided into the dry fleshed types and the moist fleshed types (these become sweeter and softer when cooked). The skin color ranges from brown, copper, pink to purple.

In marginal northern areas the choice of variety is important, as you will need to grow a short season variety such as Georgia Jet. There are also special varieties for feeding livestock that produce abundantly but aren’t so sweet.

Moist flesh types: The most popular and widely used sweet potatoes are of this type.

Beauregard

Centennial – 90 days

Covington

Georgia Jet – 85 to 90 days

Jewel

O-Henry

Puerto Rico – A compact bush type

Dry flesh types: These are mealier and not quite so popular.

Japanese

Jersey Orange

Kitchen use

The roots should be baked at 350 – 375˚F to maximize their sweetness.

Leave a Reply

Your email address will not be published. Required fields are marked *